TORONTO, July 13 (Thomson Reuters Foundation) - Brazil has not addressed the root causes of the Zika outbreak including poor sanitation and a lack of clean water access for slum residents despite declaring the public health emergency over in May, human rights campaigners and U.N. officials said.

The mosquito-borne Zika virus was linked to more than 2,500 birth defects and caused an 18-month public health emergency in South America's largest country.

But campaigners on Thursday said a lack of government investment in sanitation services for the poor, insecure water access and other conditions which exacerbated the crisis are still present in Brazil, raising the potential of future outbreaks.

"Other Zika-affected countries should recognise that human rights problems can contribute to the rapid escalation and impact of the Zika epidemic," Amanda Klasing, a researcher for Human Rights Watch, said in a statement on Thursday.

"Countries hoping to avoid the crisis Brazil continues to face should address human rights issues at the outset of their planning and response."

More than one-third of Brazil's 206 million people lack access to a continuous water supply, leaving residents with few options other than filling containers for household use.

Left uncovered, these water containers can become breeding grounds for mosquitoes and the diseases they carry.

In Brazil's poor northeast - where Zika hit hardest - less than 25 percent of the population was connected to waste water systems in 2015, Human Rights Watch said, echoing statements from U.N. officials about who was most impacted by the virus.

"Zika happens in a historical context ... it mainly affects the periphery, mainly affects brown, black and young women," said Jamie Nadal, a representative of the U.N. Population Fund during a conference earlier this year.

These historically disadvantaged groups are more likely to live in poor regions or informal settlements which do not have good healthcare and sanitation infrastructure compared to other parts of Brazil, according to campaigners.

Poor wastewater management leads to pools of stagnant, dirty water sitting in communities which are ideal breeding grounds for mosquitoes.

Across the country, more than 35 million Brazilians lack adequate facilities for the safe disposal of human waste.

Brazilian authorities say they are working to improve sanitation, especially for the country's urban poor who live in slums or favelas but a recession and political turmoil has hampered the government's investment ability. (Reporting by Chris Arsenault @chrisarsenaul, Editing by Belinda Goldsmith; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit news.trust.org)